1. Field of the Invention
The present invention relates to multiple-glazed window units of the type having a pair of sheets separated by a spacing assembly to define an airspace and, more particularly, to multiple-glazed window units having a photoelectrolytically-desiccating coating over one or more surfaces contacting the airspace. The photoelectrolytically-desiccating coating desiccates the airspace by promoting the photoelectrolysis of moisture present in the airspace and accumulated over the photoelectrolytically-activated coating into hydrogen gas and oxygen gas when the photoelectrolytically-desiccating coating is exposed to actinic radiation.
2. Description of the Related Art
Multiple-glazed window units include two or more sheets, also known as panes, of glass, plastic, metal, wood or combinations thereof spaced from each other by a spacing assembly to define a space between the sheets. Several different embodiments of multiple-glazed window units are known, and an overview of many of the embodiments are discussed in European Patent Application Publication No. 0,475,213 A1 published Mar. 18, 1992 (hereinafter "The EP Application") which is hereby incorporated herein by reference.
The airspace of the multiple-glazed window units may be sealed from the atmosphere external of the airspace (hereinafter "sealed units") or it may be in fluid communication with the atmosphere external to the unit (hereinafter "breathing units"). The term sealed units as used herein includes but is not limited to units forming a perfect or nearly perfect hermetic seal between the sheets, e.g. welded edge units as disclosed in U.S. Pat. No. 4,132,539 to Jeffries and discussed in more detail below, and units of the type employing a spacer assembly of the type which includes a spacer and an adhesive/sealant interposed between the sheets and the spacer to adhere the sheets to the spacer to form a "sealed" airspace e.g. of the type disclosed in U.S. Pat. No. 3,919,023 to Bowser et al., and U.S. Pat. No. 4,622,249 to Bowser, also discussed in more detail below. Although the latter will be referred to hereinafter as "sealed" units, as may be appreciated, such units employing an adhesive/sealant are not completely "sealed" from the external atmosphere because the adhesive/sealant used in the manufacture of such "sealed" units is not truly moisture impervious and cannot form a complete water vapor barrier. Such units permit at least a limited ingress/egress of water vapor and other gases through the adhesive/sealant and into or out of the airspace. As used hereinafter, the term "sealed" unit includes units employing a spacer and adhesive/sealant which units exhibit a water vapor permeability of less than about 20 grams/day/square meter/millimeter of mercury (hereinafter "Hg") at about 100.degree. F., 90% relative humidity as determined by the Standard Methods of Testing For Water Vapor Transmission of Materials in Sheet Form, ASTM Designation E-96-66, Method E. Conversely, as used hereinafter the term "breathing" units includes units of the type employing a spacer and adhesive/sealant, but which units further include one or more devices in addition to the water vapor permeability of the adhesive/sealant to provide fluid communication between the airspace and the atmosphere external of the unit. The term "breathing" unit also includes units having no such additional devices but which exhibit a water vapor permeability of greater than about 20 grams/day/square meter/millimeter of Hg, at about 100.degree. F., 90% relative humidity as determined by the Standard Methods of Testing For Water Vapor Transmission of Materials in Sheet Form, ASTM Designation E-96-66, Method E.
It is desirable to keep the airspace and surfaces of multiple-glazed insulated window units in contact with the airspace (hereinafter "the interior surfaces") dry, e.g. free of accumulated water or water vapor. It is also desirable to keep the airspace and interior surfaces clean e.g. free of surface contaminants. Moisture accumulated in the airspace and on the interior surfaces, particularly on glass interior surfaces, may cause fogging and may absorb or leach molecules or ions from the surfaces resulting in the formation of an undesirable scum or stain on the interior surfaces of the unit. Accumulated surface contaminants on the interior surfaces are visually unappealing.
While it is desirable to keep the airspace and interior surfaces clean and dry, it is difficult to do so because moisture and surface contaminants can accumulate in the airspace and on the interior surfaces from several sources. These sources include: entrapment within the airspace and on the interior surfaces during manufacture of the unit and/or entry into the airspace through the adhesive/sealant of units employing such an adhesive/sealant. In addition, breathing units have an additional susceptibility to the accumulation of moisture and surface contaminants within the airspace in that moisture and/or surface contaminants can enter the airspace as the atmosphere external of the unit passes into and out of the airspace.
Efforts have been made to minimize if not eliminate the accumulation of moisture in the airspace and on the interior surfaces of a multiple-glazed window unit. Some of these efforts rely upon a desiccant, and some do not.
For example, a presently available sealed unit of the type referred to as a "glass on glass" or "welded edge" unit referred to above prevents moisture accumulation in the airspace by welding edges of glass sheets to each other, and filling the airspace between the welded sheets with a dry, insulating gas. Such a unit typically does not include a desiccant. One example of such a unit is disclosed in U.S. Pat. No. 4,132,539 to Jeffries, which is hereby incorporated herein by reference.
Another presently available sealed unit interposes a spacing assembly of the type which includes a hollow spacer and an adhesive/sealant between the spacer and the sheets to provide a sealed airspace between the sheets. Such a unit includes a desiccant to absorb moisture from the airspace. Examples of such units are disclosed in U.S. Pat. No. 3,919,023 to Bowser et al., and U.S. Pat. No. 4,622,249 to Bowser, which are hereby incorporated herein by reference.
A presently available breathing unit prevents or minimizes moisture from accumulating in the airspace by associating a desiccant column with the air flowing into and out of the airspace. Such units are described in U.S. Pat. No. 2,838,809 to Zeolla et al., and U.S. Pat. No. 3,771,276 to Stewart et al.
Another presently available breathing unit prevents moisture accumulation in the airspace without requiring the use of a desiccant by including a plurality of openings in the spacing assembly to put the airspace in communication with the atmosphere external to the unit, which openings are sized and configured to allow a controlled movement of atmospheric air and moisture through the airspace. The unit equalizes the airspace pressure and relative humidity with that of the external atmosphere to prevent moisture from collecting in the airspace, yet still provides an insulating airspace. Such a unit is described in U.S. Pat. No. 4,952,430 to Bowser et al., which is hereby incorporated herein by reference.
Each of the presently available multiple-glazed window units has limitations. Insulated units having edges of the glass sheets welded together require specialized equipment to heat and fuse the edges of the sheets together. Sealed and breathing units employing a spacing assembly of the type including a spacer and adhesive/sealant are subject to ingress and egress of moisture through the adhesive/sealant, and many such units rely upon a desiccant associated with the airspace in an amount sufficient to absorb an expected rate of moisture ingress into the airspace through the adhesive/sealant over the expected life of the unit. This introduces a finite useful life for such a unit because the infiltration of relatively moist exterior air into the airspace ultimately causes saturation and exhaustion of the desiccant associated therewith. Further, surface contaminants accumulated on the interior surfaces of the presently available units can only be removed only by the complete disassembly of the unit in order to subject the interior surfaces to a cleaning operation, followed by complete reassembly of the unit.
All of the above-discussed presently available multiple-glazed window units are acceptable in one or more applications; however, as can now be appreciated, not every unit is ideally suitable for every use. It would be advantageous therefore to provide a multiple-glazed window unit having features which make the unit less expensive to manufacture than the presently available units while eliminating the limitations of the presently available units.